In the field of wireless communication, the terms “User Equipment, UE” and “wireless device” are commonly used for various communication entities e.g. including mobile telephones, tablets and laptop computers. In this disclosure, “wireless device” will be used to represent any wireless communication entity capable of communicating radio signals with a wireless network. It should be noted that a wireless device in this context may also be a Machine Type Communication, MTC, device such as a sensor, counter or measuring device arranged to operate automatically and send reports or other messages to some central node.
Further, the term “transmission point”, sometimes also referred to as a base station, network node, radio node, eNodeB, eNB, etc., represents any node of a wireless network that is arranged to communicate radio signals with wireless devices. Throughout this disclosure, the term transmission point is interchangeable with eNodeB, eNB, Tx/Rx point or base station, and the term wireless device is interchangeable with UE.
Long Term Evolution, LTE, is a mobile broadband wireless communication technology defined by the third Generation Partnership Project, 3GPP. According to LTE, radio signals are transmitted from transmission points referred to as eNodeBs or eNBs by 3GPP, to wireless devices referred to as UEs by 3GPP, using Orthogonal Frequency-Division Multiplexing, OFDM. In a wireless network, a Time Division Duplex, TDD, configuration of subframes may be used for uplink and downlink transmissions in cells where consecutive subframes are comprised in a repeatable radio frame.
The subframes are reserved for either uplink transmissions from wireless devices to a serving transmission point or downlink transmissions from the transmission point to the wireless devices such that uplink and downlink transmissions do not occur at the same time within the cell. A subframe is basically defined by a preset time period of a certain length, typically 1 millisecond (ms), and each subframe may comprise two time slots of 0.5 ms each. Further, a radio frame comprises a predefined number of consecutive subframes, e.g. ten subframes. In such a network, different transmission points are able to use different uplink-downlink, UL-DL, configurations of subframes, e.g. depending on the current need for uplink and downlink bandwidth resources. An example of a downlink-uplink configuration of subframes is shown in FIG. 1. including downlink subframes “D”, uplink subframes “U” and so-called special subframes “S”. The special subframes S are configured with one part reserved for downlink, another part reserved for uplink, and a guard period with no transmission between the above two parts, thus allowing neither uplink nor downlink during the guard period.
A set of different predefined UL-DL configurations that can be used by transmission points in different cells is shown in the table of FIG. 2 comprising seven different UL-DL configurations 0-6 each having ten subframes 0-9 of a repeatable radio frame It can be seen in this example that the first three subframes 0-2 and subframe 5 are reserved for downlink D, special S, uplink U, and downlink D, respectively, in all UL-DL configurations 0-6, while the remaining subframes 3, 4, 6-9 can vary in different UL-DL configurations. The latter subframes 3, 4, 6-9 may thus be referred to as flexible subframes, indicated by “F” in the figure, and the other subframes are typically called fixed subframes.
In this disclosure, the term “flexible subframe” denotes a subframe in which the direction of transmission, i.e. downlink or uplink, may differ between different cells so that the flexible subframe may be used for downlink in one cell and for uplink in another cell. Further, a flexible subframe may differ from one radio frame to another in the same cell so that the flexible subframe may be used in the cell for downlink in one radio frame and for uplink in another radio frame, which is commonly referred to as “dynamic TDD”. Thereby, the link direction of transmissions in flexible subframes is not obviously predictable and some amount of signaling is needed from the cell's transmission point to indicate the current direction of a flexible subframe to wireless devices in the cell.
However, it may be a problem that when a wireless device is operating in a Coordinated Multi Point, CoMP, scenario and is served by two or more transmission points, it may receive at least two different UL-DL configurations signaled from different transmission points where one or more subframes are flexible in the sense that they do not have the same link direction in all serving transmission points. In other words, the wireless device may receive UL-DL configurations with at least one conflicting subframe which is configured for uplink transmission in one serving transmission point and for downlink transmission in another serving transmission point, e.g. such as any of subframes 3, 4, 6-9 in FIG. 2. Consequently, the wireless device is not able to determine in which subframes it should monitor a downlink control channel and possibly also perform Channel State Information, CSI, measurement which in turn may result in missed control information such as resource assignments and possibly also deficient measuring of channel quality.